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Hi Mike and Dennis,

I worked at Flandrau for a long time and can give you a little 
information about their heliostat and the problem in general.

Basically, in Flandrau's design, there is an equatorial mount on the 
roof of the building - main axis parallel to the Earth's axis; ei, 
roughly pointing toward Polaris. At the lower end of the axis is a fork 
that holds the primary mirror. In their case the primary mirror is an 
optical flat about 12" in diameter and about 2" thick. The fork allows 
the primary mirror to aim the sun right down the axis, through a hole in 
the ceiling and into the building.

At this point the sunlight is theoretically stationary. You can capture 
the beam, bend it with a good first surface mirror, pass it through 
imaging system, prisms, diffraction grating, what ever. If imaged, the 
face of the sun will rotate throughout the day. To eliminate this 
rotation there is a more sophisticated device, I believed it's called a 
celeotstat, which does keep the image's orientation the same throughout 
the day.

When it was properly pointed it had to be adjusted at least once a day - 
to correct for the sun moving up or down seasonally. This was not much 
of an issue. Clock motors on the Declination axis and push buttons on 
the exhibit floor made it easy. Of course, later in its life, when it 
wasn't so accurately pointed, it had to be adjusted several times a day. 
Nowadays, this could probably all be taken care of electronically with 
some kind of feedback loop.

If you are not going to image the sun the quality of the optics are not 
so important. If you are going to image you need very good quality 
optics. The cost of a large optical flat can be ... astronomical.

Also, if you image, you need to eliminate all vibration. The light is 
thrown a long distance. The tiniest movement will be greatly 
exaggerated. If you just want to aim the light at a prism or diffraction 
grating then it's not such a big deal.

I made a crude heliostat for the Children's Discovery Museum of San Jose 
out of Unistrut and the drive shaft of an old Chevy. It sat on the 
ground on the South side of the building and shot light into a window on 
the second floor. It wasn't very accurately pointed or made, but it 
worked well enough for a while.

Any 'astronomer' can give you a better description of this. And in case 
you can't tell, I'm no astronomer!

Joe Ruggiero
www.TheExhibitGuys.com


Mike Shanahan wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
> 
>  
> 
> We are trying to find a way to project the sun into a museum building
> and on to a prism to create a rainbow effect.  Does anyone have advice
> on what it takes to get a tracking device that would work year-round to
> project the sun's light into a building?  We of course have telescopes
> that can be set up to do a similar job on a daily basis but this would
> be a more permanent installation and one that would, ideally, not
> require daily adjustment.  
> 
>  
> 
> I recall seeing a device at the space museum in Rio de Janeiro that
> projected the sun onto the museum floor...I should have taken more
> notes!
> 
>  
> 
> Reply online or offline, if you would.
> 
>  
> 
>  
> 
> Mike Shanahan
> 
> Education Director
> 
> Bishop Museum
> 
> 1525 Bernice Street, Honolulu HI 96817
> 
> Office: (808) 847-8235
> 
> FAX: (808) 847-8250
> 
> Cell/pager: (808) 386-1419
> 
> www.bishopmuseum.org
> 
>  
> 
> 
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